Generally, the conventionally-known vertical engines include a cylinder section provided beside a crankcase accommodating a crankshaft oriented in a substantially vertical direction, a cylinder head overlappingly fixed to a lateral side of the cylinder section with a sealing gasket interposed therebetween, and a valve chamber provided in the cylinder head. The vertical engines also include an oil pan in a bottom portion of the crankcase so that lubricating oil stored in the oil pan can be supplied via an oil supply pump from the oil pan to the valve chamber via oil supply passageways, and the lubricating oil remaining in the valve chamber is returned to the oil pan via an oil return passageway. Further, the valve chamber is in air communication with the interior of the crankcase via an air passageway, a check valve is provided in the air passageway for permitting passage therethrough of air only from the interior of the crankcase to the valve chamber, and a resilient pressing section is provided around the air passageway.
One example of such conventionally-known vertical engines is disclosed in Japanese Patent Application Laid-Open Publication No. 2005-48722 (hereinafter referred to as “relevant prior patent literature”), which can enhance durability of the check valve.
A lubrication device for the valve chamber of the vertical engine disclosed in the relevant prior patent literature is constructed, for example, as one that compulsorily pressure-feeds or force-feeds lubricating oil by use of an oil pump, one that directs oil mist, contained in an atmosphere within the crankcase, to a tappet chamber by use of gas flows to a breather provided in an upper portion of a combustion chamber (valve chamber or tappet chamber), or one that directs oil mist within the crankcase by appropriately setting respective sectional areas of two guide passageways, provided between the crankcase and the tappet chamber, to produce a phase difference in inner pressure variation between the crankcase and the tappet chamber, or the like.
For example, in the case where lubricating oil is force-fed by use of the pump, the capacity of the oil pump has to be increased due to increase in the number of oil lubricating paths and increase in component parts to be lubricated, which would undesirably lead to increase in pumping power loss.
Further, in the case where gas flows, within the crankcase, by the breather are used, it is necessary to meet both of conflicting requirements of directing an amount of oil necessary for lubrication and of minimizing oil in discharged gas from the breather from a perspective of efficient oil consumption, which would however be very difficult in view of a layout of various component parts.
Furthermore, in the case where a phase difference in inner pressure variation is produced between the crankcase and the tappet chamber, an amount of lubricating oil in the tappet chamber too may fluctuate due to operating condition of the engine because the inner pressure variation tends to be unstable due to complicated factors, such as an amount of lubricating oil within the crankcase, an amount of blow-by gas and the number of rotations of the engine.